1. Field of the Invention
The present invention relates generally to skin friction force measuring devices and, more specifically, to a skin friction balance having a sensing element mounted on a parallel linkage mechanism.
2. Description of the Related Art
Skin friction balances are generally known. A skin friction balance is used to determine the skin friction drag component of the total drag acting on an aerodynamic body. A variety of balances have been devised to address certain problems which affect the accuracy of the skin friction measurement. In U.S. Pat. No. 2,935,870 to Lyons, a skin friction balance is described which avoids inaccuracies which result from acceleration forces. The apparatus of U.S. Pat. No. 2,935,870 includes a linear variable differential transformer having stationary coils and a movable core. The core is mounted to a central body which is movable relative to a stationary base when skin friction force acts upon a surface element attached to the central body. A counter-weight system prevents skin friction force measurements from being effected by linear and angular accelerations.
U.S. Pat. No. 3,304,775 to Kistler describes a skin friction gage for use in a high speed, high temperature environment, such as what is experienced by high Mach speed re-entry vehicles. A small sensing element constitutes a portion of the surface where it is desired to measure skin friction drag and is free to move relative to the surrounding area. Movement of the sensing element causes a corresponding motion of a differential capacitive pickoff. A change in differential capacitance produces a corresponding current through a coil mechanically attached to the pickoff and the sensing element. A force is created which opposes the outside forces which tend to cause displacement of the sensing element. Thus, measurement of the current is equivalent to measurement of the external drag force.
U.S. Pat. No. 4,240,290 to Montoya et al. describes a skin friction measuring device which uses a potentiometer, the power supply of which is varied in accordance with a variable resistor. An air stream passing over the surface of a friction plate causes displacement of the plate and deflection of flexural members supporting the friction plate.
U.S. Pat. No. 3,383,914 to MacArthur teaches a skin friction transducer which addresses the problem that inaccurate measurements result when the sensing element experiences pressure force normal to the sensing area. To solve the problem, fluid communication was provided between opposite sides of the sensing element to equalize pressure.
U.S. Pat. No. 4,604,903 to Tcheng et al. describes a skin friction balance which is capable of two-axis self-nulling. Skin friction forces were measured along two perpendicular axis by providing an L-shaped arm having a second arm pivotally connected to one end thereof. Two separate nulling devices were provided to keep a sensing element in the null position.
The above-described known devices are generally sensitive to off-center normal force and/or acceleration. Also, because most of the above are "open-loop" devices, accuracy is generally less than acceptable. Most open-loop designs require a larger than acceptable gap size between a balance case and the sensing element.